I'm almost convinced that the compressor on my 1984 300SD runs whenever one of the three buttons on the left side (Defrost, down and up arrow, and down arrow) of the climate control unit are pressed. It's a little hard to tell when driving so I'm going to wire a light in series so I can see when the compressor clutch is engaged.

If it is on all the time as I suspect, is this correct? If not, are there any hints on where to start looking? The A/C system seems like a black hole to me.

Thanks!